Plane polarized lenses may be used in sunglasses that are specific to activities at or near the surface of water to reduce the glare of the reflected sun light from the water's surface. Generally speaking this has the double purpose of allowing greater visibility to events or objects below the surface of the water, as well as to reduce stress on the eyes of the observer or sensors of the observation equipment.
Similar effects and uses are found elsewhere where reflective surfaces abound, such as in snow-covered landscapes or driving. Note that this is not a question of generally darkening the landscape but rather of removing selected light which is incident from uninteresting reflections (glare) rather than from objects that are being viewed.
Commonly available sunglasses include lenses of uniform polarization meaning that all points of the lens block light that is polarized in the same plane. This specific plane is chosen to absorb the most light possible in the most general case possible.
For example, FIG. 1 illustrates a polarized set of sunglasses including a common polarization angle throughout the lens. In this case, glare is blocked from reflections based on the directional source of the light because specific sources can reflect from surfaces that will create this polarization angle once reflected.